The human body is equipped to adapt to the increased need for cardiac output during exercise. If the heart is functioning properly, the nervous system increases the heart rate and reduces peripheral resistance in response to exercise. However, a large, increasing population of patients have pacemakers to compensate for various heart conduction disorders. With rate adaptive and exercise-responsive cardiac pacemakers being developed, the pacemaker has not only become a life sustaining device for a significant number of people with cardiac conduction problems, but it has also become a device for improving the quality of life for these patients to lead a more normal existence.
Several physiological measurements have been utilized to indicate pacing rates during exercise. These physiological parameters include pH, QT interval, respiratory rate, body motion, and the venous blood temperature in the right ventricle of the heart. Blood temperature is a good physiological indicator for several reasons. During exercise, muscles generate considerable heat, causing an increase in the temperature of venous blood. Factors other than exercise such as emotional anxiety also cause an increase in blood temperature. Even the anticipation of activity causes a brief decrease in the temperature of mixed venous blood.
A typical blood temperature response to activity or stress from a baseline temperature initially includes a sudden and brief decrease in temperature, which is caused by peripheral vasodilation at the onset or anticipation of exercise. This temperature drop is followed by a rise in temperature or by a brief leveling off period and then a rise in temperature. If the activity is brief, the temperature will slowly return toward the baseline temperature. If the activity or exercise continues, the temperature rises more rapidly and continues to rise until the activity ceases. After exercise ceases, the temperature returns toward the baseline.
Temperature profiles vary among patients. The magnitude of each component of the temperature response varies with the individual patient and the level of activity. Some patients tend to exhibit a pronounced drop, while others exhibit no drop or only a slight temperature drop. More strenuous activities cause a faster rise in temperature. Also, repeated activity during a short time period tends to reduce the magnitude of the drop by decreasing the temperature difference between central and peripheral circulation.
One temperature-based, rate-adaptive pacemaker is the KELVIN.TM. 500 pacemaker manufactured by Cook Pacemaker Corporation of Leechburg, Pa. This pacemaker includes a pacing algorithm that controls the stimulation rate of a pulse generator between programmable lower, interim, and upper stimulation rates; which are normally associated with resting, brief exercise, and continued exercise conditions, respectively. Sensitivity to temperature changes and the speed of stimulation rate changes are separately programmable. This pulse generator recognizes the initial decrease in temperature at the onset of activity and increases the stimulation rate to the programmed interim rate appropriate for short duration activities. This provides an early increase in the stimulation rate, while limiting further rate increases until sustained activity is confirmed by a temperature rise. This is also a safety feature which limits the stimulation rate to the programmed interim rate until a continued temperature rise is confirmed.
The pulse generator stimulates the heart at the interim rate for a programmed interim time period if a sufficient temperature rise to indicate continued activity does not occur. Basically, this interim time period allows for stimulation at the interim rate during short bursts of activity, which are generally unaccompanied by a sufficient temperature rise for stimulation at the upper rate. When the activity is brief and does not cause a sufficient temperature rise, the stimulation rate of the generator returns to the programmed lower rate after the programmed interim time period has elapsed or terminated. When sustained exercise is confirmed by a sufficient positive temperature rise over time, the stimulation rate of the generator is increased to the programed upper rate. When exercise ceases, the temperature returns toward the baseline temperature, and the stimulation rate returns to the lower rate. When the post-exercise temperature decreases rapidly, the stimulation rate may be maintained at the interim rate for the interim time period before returning to the lower rate, which provides additional cardiac output.
The predetermined interim time period is programmable presently from 2-12 minutes in 2 minute increments with an interim time period of six minutes being satisfactory for most patients. However, even with an interim time period of six minutes, patients have complained of the elevated stimulation rate for the entire interim time period after experiencing mild or brief exercise such as sitting or standing up from a prone position. Since the interim time period is selected to meet the overall needs of the patient, it would be desirable to modify the interim time period based on the real time exercise level of the patient.